1. Field of the Invention
The present invention relates to a focus detection method and a focus detection apparatus of a digital camera.
2. Description of the Related Art
A contrast detection method is known as a conventional focus detection method for compact AF digital cameras. Conventional focus detection apparatuses using the contrast detection method operate to determine the position of a focusing lens group in which contrast of an object image captured by an image pickup device becomes maximum. Therefore, in such conventional focus detection apparatuses, an imaging operation is performed while a focusing lens group is driven stepwise from the infinite focus position (position for bringing an object at infinity into focus) to the closest (shortest) focus position to obtain a contrast from a captured image signal at each different position of the focusing lens group, and it is determined that a peak value of contrast among the obtained contrasts corresponds to a focused focal point, so that the focusing lens group is driven to a position thereof where the peak value of contrast is obtained to bring the object into focus. Such a focus detection apparatus is disclosed in, e.g., Japanese Unexamined Patent Publication No.2001-249267.
In the contrast detection method, a difference in contrast which is obtained by moving the focusing lens group in the optical axis direction is small in the case of a low-contrast object. Nevertheless, in the conventional contrast detection method, the reliability of the detected peak value of contrast is low because a reversal point where either the contrast (contrast value) of a captured image commences to decrease after increasing more than once (see FIG. 13A), or the contrast of a captured image commences to decrease more than once after increasing (see FIGS. 13B and 13C), or because only one of the number of successive increments of the number of successive decrements is counted to detect a peak value of contrast.